1. Field of the Invention
The invention concerns an apparatus for supporting at least a portion of a human or animal body with cooling localized at or focused on the weight-bearing areas of the portion. The localized, or focused, cooling reduces the risk of damage to the weight-bearing body areas of patients confined to beds or wheelchairs for long periods of time.
The application of pressure to the skin of a patient for a prolonged period of time has been known to cause pressure ulcers or pressure ulcers. The weight-bearing areas of the body surface are exposed to pressures that can easily exceed 100 mmHg (torr.). It has been shown that blood flow ceases in capillaries that are exposed to compressive pressures exceeding 25 torr. Therefore, the weight-bearing areas of the body surface and subcutaneous tissue can be expected to have inadequate blood flow or even a complete lack of blood flow during the time the weight is borne.
Normal cellular metabolism depends on adequate circulation of blood to deliver oxygen, nutrients and to remove waste products. Prolonged interference with the local circulation results in a two-part sequence of events, beginning with ischemia (a severe reduction of blood and oxygen supply to the tissues), and terminating in necrosis (irreversible death of the cells and tissues, resulting in sloughing).
People normally will shift their positions in a chair or get up and walk around to relieve the pressure on their buttocks. People normally will regularly roll over in bed while sleeping to periodically redistribute their weight to a different surface area. This movement is usually in response to pain or discomfort caused by the tissue ischemia of the weight-bearing skin, subcutaneous or deeper tissue. Bed-ridden or wheelchair-ridden patients or patients on operating tables may not be aware of the ischemic pain if they have brain or spinal cord injury, stroke injury, dementia, prolonged surgery under anesthesia, or prolonged sedation and mechanical ventilation. Alternatively, severe illness, neuro-muscular diseases or nervous system injury may prevent patients from moving even if they are aware of the ischemic pain.
The incidence of pressure ulcers in surgical patients varies from 12% to 66% in different studies. Surveys of general hospital patients indicate that 3-4.5% of all patients develop pressure ulcers during hospitalization. Pressure ulcers usually develop near regions of the body which have a bony prominence near the skin. More than 80% of all pressure ulcers occur at the following five locations:
1. Sacro-coccygeal region (high buttocks), supine position. PA1 2. greater trochanter (low hip), lateral recumbent position. PA1 3. Ischial tuberosity (low buttocks), sitting position. PA1 4. Tuberosity of the calcaneus (heel), supine position. PA1 5. Lateral malleolus (outer ankle), lateral recumbent position. PA1 1. Regular turning or movement of the patient to minimize the duration of time that pressure is applied to any give surface area. The tissue is allowed time to re-perfuse during the period that the pressure is not applied. PA1 2. Passive support surfaces (cushions, mattresses and pads of all kinds), which may utilize unique or special materials or shapes to minimize the pressure exerted against any given point of the body surface. Many types of materials have been tried including; different types of polymeric foam, polymeric gels, water and air filled bladders. PA1 3. Active support surfaces such as a series of air filled bladders that alternately inflate and deflate to automatically redistribute the pressure. PA1 1. Even mild heat will accelerate the rate of tissue injury due to pressure induced ischemia. PA1 2. Mild cooling will protect tissue from injury due to pressure induced ischemia.
Surgical and bed ridden patients are not the only patients susceptible to pressure ulcer development. For example, paralyzed patients spend much of their lifetime in a wheelchair. One study indicated that the incidence of pressure ulcers is 21.6% for paralegics and 23.1% for quadriplegics.
Equipment for pressure ulcer prevention has focused in three areas:
Considering the high incidence of pressure ulcers despite the availability of these many passive support surfaces (various materials and shapes), it is clear that simply distributing the pressure to a larger surface area, in and of itself, will not effectively prevent pressure ulcers. Although active support surfaces have been used to prevent pressure ulcers, it is clear however that they are very expensive, cumbersome and noisy. Therefore active support surfaces are not likely to be used in the majority of pressure ulcer prevention situations.
Finally, aside from the obvious pain and health risk to the patient (having a chronic infection in a chronic open wound), pressure ulcers are extremely expensive and slow to heal. Healing the average pressure ulcer costs $30,000 to $40,000 and takes about 3 to 6 months. The high incidence of pressure ulcers, the lack of any proven method of preventing pressure ulcers and the extremely high cost of healing a pressure ulcer once it develops, clearly indicates a significant need for a new technology.
It is reasonable to assume that heat should be an important factor in the formation of pressure ulcers. All tissues increase their metabolic rates 7-10% for each 1.degree. C. increase in temperature. The increased metabolic rate increases the demand of the cells for oxygen a similar 7-10% for each 1.degree. C. increase in temperature. In a patient whose tissue perfusion is already compromised by external pressure or by vascular insufficiency, this increased metabolic demand for oxygen could increase the rate of tissue injury. We hypothesized that this increased metabolic demand was the cause of the frequent "burns" observed after water mattress warming therapy during surgery, despite the relatively low temperatures (39.degree.-42.degree. C.) of the mattresses. These low temperature injuries may result in full thickness skin damage which appears identical to third degree burns resulting from exposure of the skin to high temperatures. While the full thickness damage to the skin is identical to a high temperature thermal injury ("burn"), in reality the injury is caused by pressure necrosis which is accelerated due to the increased metabolic rate of the tissue. While this interrelationship between temperature, pressure and tissue ischemia is scientifically logical, it had never been proven prior to our recent experiments.
Further, it is known that hypothermia decreases the cellular metabolic rate and increases the tolerance of cells to periods of inadequate blood flow. This is the reason that patients are cooled during cardiac bypass. We therefore hypothesized that cooling the skin and subcutaneous tissue would effectively prolong the time to injury, in the face of the ischemia caused by an inadequate local blood flow resulting from pressure exerted against that tissue.
To test these hypotheses, we developed a porcine model to investigate pressure ulcer formation. Twelve metal discs were applied to the back of an anesthetized swine. The pressure on the skin under each disc was approximately 100 torr (totally occlusive to blood flow), for a 10 hour period of time. The temperature of the discs was carefully controlled at 25.degree. C., 35.degree. C., 40.degree. C. and 45.degree. C. Normal porcine temperature is 38.degree. C. (Normal human body temperature is 37.degree. C.) The severity of the resultant tissue injuries directly correlated with an increase in temperature. No tissue damage was found under the 25.degree. C. discs. Severe damage of the skin, subcutaneous and deep tissues was found under the 45.degree. C. discs. The 35.degree. and 40.degree. C. discs also caused severe damage, but intermediate to the extreme temperatures. The results of this experiment strongly indicate that both of our hypotheses were correct:
Water mattresses circulating cool or even cold water have been used for decades to cool febrile patients. However, experience shows that the application of cold to widespread surface areas of the body is both extremely uncomfortable and will cause hypothermia and shivering.
2. Description of the Related Art
In the prior art, U.S. Pat. No. 3,738,702 discloses a seat structure that cools a portion of the human body that rests against the seat in response to the heat of the body, where the body engages the seat. In order to maximize the sensitivity of the cooler to body heat, the cooler is placed as near as possible to the surface of the seat contacted by the body.